Embodiments described herein relate generally to devices, systems and methods for measuring electrodermal activity, and in particular to wearable devices and methods for measuring electrodermal activity of the skin of a user.
The human skin is composed of different layers of tissue. These layers of tissue perform several functions, for example, forming an interface between the internal and external parts of the body, serve as a protection mechanism, have a thermoregulatory function, and allow exchange of fluids through the skin. The human skin also includes sweat glands that produce sweat. The sweat includes various electrolytes which allow current to be conducted through the skin. For example, if two electrodes capable of producing free ions such as, for example, silver (Ag) electrodes are disposed on the skin, free ions can be electronically communicated between the two electrodes via the skin.
The conductance of skin, which is generally referred to as the electrodermal activity, is extremely low and is generally measured in Siemens (S). The conductance of the skin depends upon the thickness of the stratum corneum. The inner layer of the skin creates a potential barrier which changes in size and allows the current to flow in a less or more restricted way in the stratum corneum. The thinner the stratum corneum, the higher is the conductance. For example, the conductance of skin at the finger tips can be in the range of about 0.5 μS to about 50 μS, and the conductance of the skin at the wrist can be in the range of about 0.05 μS to about 80 μS. These variations can depend on many factors, including the physiology of an individual, temperature, skin structure, and autonomous nervous system (ANS) activity.
The electrodermal activity signal generally includes two interleaved signals; the tonic level and phasic level. The tonic level (also referred to herein as “tonic level conductance”) is the skin conductance in the absence of any external or environmental stimuli, is slow changing (i.e., low frequency), and is caused by the human physiological factors as described herein. The tonic level can have a range of about 0.05 μS to about 50 μS at the wrist of a user.
The phasic level (also referred to herein as “phasic level conductance”) is typically associated with short-term events and occurs in the presence of discrete environmental stimuli such as for example, sight, sound, smell, and cognitive processes that precede an event such as anticipation, decision making, etc. Phasic changes usually show up as abrupt increases in the skin conductance, or “peaks” in the skin conductance.
Systems and devices can also be used to measure heart rate variability (HRV) through the skin, or in the blood beneath the skin of the user. The HRV is defined as the beat-to-beat variations in heart rate. The larger the alterations, the larger the HRV. HRV is a known predictor of mortality of myocardial infarction and other pathological conditions may also be associated with modified (usually lower) HRV, including congestive heart failure, diabetic neuropathy, depression post-cardiac transplant, susceptibility to sudden infant death syndrome (SIDS), and poor survival in premature babies. HRV is also related to emotional arousal. HRV has been found to decrease during conditions of acute time pressure and emotional strain, elevated levels of anxiety, or in individuals reporting a greater frequency and duration of daily worry.
HRV includes two primary components: respiratory sinus arrhythmia (RSA) which is also referred to as high frequency (HF) oscillations, and low frequency (LF) oscillations. HF oscillations are associated with respiration and track the respiratory rate across a range of frequencies, and low frequency oscillations are associated with Mayer waves (Traube-Hering-Mayer waves) of blood pressure. The total energy contained by these spectral bands in combination with the way energy is allocated to them gives an indication of the heart rate regulation pattern given by the central nervous system, and an indication of the state of mental and physical health.
However, known methods for analyzing heart beat data to determine HRV and a psychophysical state of a person often fail to determine a true mental and physical state of the person. Some known HRV spectral analysis methods use non-parametric approaches (e.g., Fast Fourier transforms) or parametric approaches. These strategies rely on the approximation that the tachogram is “sampled” at a constant frequency. Such known methods are susceptible to missing beat data or high variability in the heart beat data. Furthermore, high activity can also lead to high variability in the heart beat data which cannot be analyzed properly by known methods.
Thus, there is a need for new systems, devices and methods that can measure skin conductance with high reliability, repeatability and do not suffer from electrolysis. Furthermore, there is also a need for new methods to analyze heart beat data and determine human well being through heart rate variability.